Light-water-cooled nuclear reactors, and in particular nuclear reactors cooled by pressurized water, use nuclear fuel assemblies comprising fuel rods which extend along the longitudinal direction and which are held in a bundle by a skeleton assembly. Each rod comprises cladding within which the nuclear fuel pellets are stacked.
The skeleton assembly of a nuclear fuel assembly in particular comprises a plurality of spacer grids distributed over the length of the bundle of rods.
Spacer grids ensure that the rods are held in place transversely and each comprise a set of adjacent cells arranged in a substantially regular array, generally a square grid. The cells themselves have a cross-section of square shape in a plane transverse to the spacer grid.
Each of the cells is delimited and separated from adjacent cells by a peripheral belt of walls having the shape of the lateral surface of a rectangular solid on a square base.
The cells of the spacer grid are open at their two longitudinal extremities so as to receive a rod in a generally central arrangement, in which arrangement the axis of the rod is located along the axis of the cell.
Furthermore, the nuclear fuel rods have a diameter which is substantially smaller than the size of the cells, so that there is a free space around each rod, between the outer surface of the rod and the peripheral wall of the cell in which the rod is longitudinally fitted.
In each of the cells intended to house a fuel rod the spacer grid comprises supporting members which are intended to come into contact with the outer surface of the corresponding fuel rod. The supporting members hold the rod in place in transverse directions perpendicular to the axis of the rod and the cell and in the longitudinal direction, while permitting limited longitudinal movement of the rod, for example through the effect of expansion within the core of the nuclear reactor.
The supporting members comprise rigid dimples and/or resilient dimples and/or springs.
These members project within the cells with which they are associated and are made by cutting and pressing out the peripheral belts of the walls, or are attached to and secured to the latter.
Supporting members which are orientated longitudinally, as for example described in document U.S. Pat. No. 5,793,832, or transversely, as described in document U.S. Pat. No. 5,183,629, or again obliquely, as for example described in document U.S. Pat. No. 4,803,043, are known.
Each supporting member comes into contact with the outer surface of the corresponding fuel rod through the surface of a contact part, this surface being orientated towards the interior of the cell in question.
This inner surface is generally flat so that the contact with the nuclear fuel rod is theoretically a cylinder/plane contact.
When the nuclear reactor is in operation, the cooling water, which circulates in the core with a large upward velocity, causes small amplitude oscillating movements in the nuclear fuel rods within the spacer grids. This phenomenon, known as “fretting”, will result in friction between the nuclear fuel rods and the contact parts which may give rise to wear of the nuclear fuel rod cladding. This wear may cause the fuel rod cladding to be pierced and therefore gases and radioactive materials to be released into the water of the primary circuit, which may result in shutdown of the reactor for the premature removal of fuel assemblies which include defective rods.
In order to reduce these risks, document U.S. Pat. No. 5,243,635 describes a grid of the aforesaid type. The inner surfaces of the contact parts of the supporting members have transverse concavity of the same radius of curvature as the outer surfaces of the rods. This transverse concavity makes it possible to envelop the outer surface of the nuclear fuel rod cladding and thus increase the area of the effective contact areas with the rods and therefore reduce the risk of damaging the cladding by fretting.
However, differences from nominal dimensions and positions which necessarily arise in manufactured spacer grids because of manufacturing tolerances result in changes in the relative positions of the parts in contact in relation to the outer surfaces of the nuclear fuel rods. Contact can therefore be established in an uncontrolled way, for example through the lower, upper or side edges of the contact area, in particular when the surface of the contact part is not tangent upon the surface of the fuel rod, and this can lead to rapid damage of the rod cladding by fretting.